DE4391398C2 - Method and device for detecting a lack of coolant in an air conditioning system - Google Patents

Description

The present invention relates to technology for the precise detection of a lack of a coolant in an air conditioning system for use in refrigerators for Automobile or the like is suitable.

An air conditioning system essentially uses a coolant sensor in a closed circuit in which the coolant circulated, installed to detect whether the amount of Sufficient coolant in the closed circuit or is not sufficient. The present invention proposes a coolant sensor shown in the non-prepublished Japanese Patent Publication No. 5-180546 (hereinafter referred to as prior art), the one sensor body with a coolant flow path section for the passage of the coolant in a flow path between a condenser and an evaporator in one Cooling circuit is provided, a coolant chamber, which in the sensor body above the coolant flow path section is arranged and with the interior of the coolant flow path cut communicates, and a heat sensitive Component that is inside the coolant chamber for detecting whether the coolant inside the chamber in a liquid to stand is, is provided, includes.

The coolant sensor, which is constructed as above, is in a pipe between a collection tank that holds the liquid Coolant con was contained, and between an expansion vein til, whereby the liquid coolant expands and evaporates is fitted. With the driving of the air conditioning system goes Compressor in operation, and if that in the refrigeration cycle circulating coolant to a sufficient amount  is filled, the inside of the coolant flow path section of the coolant sensor with the liquid coolant that is there flows through, filled until the surface of the liquid reaches the top surface of the coolant chamber interior.

After the heat sensitive component inside the cooling middle chamber in which liquid coolant is immersed at this time due to the coolant be interpretative cooling effect such that the resistance value of Components such as B. a thermistor (thermistor) from which the heat-sensitive component is manufactured on a low temperature value is maintained. If the crowd of the coolant in the cooling circuit, however, due to a Reduced coolant leaks, etc., the liquid drops level of the coolant inside the coolant chamber and that heat-sensitive component lies above the coolant liquid surface free. After the cooling effect of the gas shaped coolant is less than that of the liquid Coolant, the temperature of the heat sensitive rises Component, with a consequent change in the contra standes that a change in the current caused by a warning device is running, causing a light to turn on which indicates the lack of coolant.

With the coolant sensor according to the prior art a movement of the liquid surface of the coolant in the coolant chamber due to fluctuations in pressure of coolant in the pipeline caused by a change the compressor speed (for a compressor for a car mobile due to a change in engine speed, the drives the compressor), even without changes the coolant quantity can be increased or decreased Lich. As a result, the level of the liquid coolant surface area greater than or less than that of the heat-sensitive component, the component with a subsequent change in the cooling effect or is exposed.  

Therefore, if a lack of coolant is detected with a coolant sensor, the z. B. uses a thermistor with a high resistance Wi at low temperatures as a heat sensitive component, then the characteristics as shown in Fig. 6 are obtained. In this figure, the compressor speed is represented by the horizontal axis, the current I in the warning device circuit is represented by the vertical axis and the characteristic curves B10, B20, B40, B60, B80, B100 and B120 show the characteristics for the respective coolant level Percentage rates in the range of 10, 20, 40, 60, 80, 100 to 120%, where 100% is ideal. The current value 10 in FIG. 6 is the value for the case when the warning device activates a lamp or the like in order to issue a warning about insufficient coolant.

With such a coolant sensor, even if there is a sufficient fill level in the cooling circuit 1 , at a coolant fill percentage F of z. B. 60%, as shown by the characteristic line B60, reports a lack of coolant due to a deficiency condition when the compressor speed N is less than about 2000 rpm. However, increasing the engine speed incorrectly detects a sufficient condition and clears the low-coolant warning. Moreover, in the case at the extreme end of the graph with the characteristic line B10 for the low coolant level percentage, then regardless of the fact that there is a low coolant condition, it is not possible to judge accurately whether a sufficient condition or deficiency condition exists when the compressor speed N is above about 5000 rpm.

JP-A-3-59371 relates to a detection device a coolant leak of a cooling cycle. This well-known Device works in such a way that after commissioning a compressor as soon as this operating current stabilizes is whose load current is detected by a current detector.  

The load current is fed to a microcomputer which Compares the load current with a reference load current that corresponds to the Load current of the compressor corresponds when the cooling cycle is filled with a normal coolant. If determined becomes that the load current is smaller than the reference load current, then leakage of the coolant is confirmed and the compressor stops operating. The device to detect a coolant leak, an "indirect" Detection of a coolant leak through. This indirect Detection takes place via the load current of the compressor.

JP-A-3-236572 discloses moisture detection direction in a cooling cycle. In this well-known Vorrich device, a moisture sensor generates an output signal that is output to a control circuit. The control circuit compares the humidity during the operation of a comm pressors with the moisture after this stopped has been. Depending on the size detected, an alarm signal is generated issued when it is determined that the moisture content is abnormal (too high).

JP-A-2-103357 describes a detection device for detection a vapor-liquid ratio.

The present invention is based on the object Method or device for detecting a defect of a coolant in an air conditioner to create a accurate detection regardless of a compressor speed enable, and to an unloaded operation of the Verdich ters and problems such as B. seizing or blocking of the compressor.

This object is achieved by a method according to claim 1 and solved by a device according to claim 2.

Advantageous developments of the present invention are defined in the dependent claims.  

With the above structure, the output signal of the coolant shows telsensors that the amount of coolant is the actual The coolant level exceeds if the amount of liquid coolant that is in contact with the coolant sensor is dealing with an increase in compressor speed elevated. After the evaluation value, that of the compressor speed corresponds, but is set to the amount to compensate for the change in the coolant sensor output value Chen, the comparison of the sensor output with the Judgment value that a coolant deficiency state with high Accuracy recorded without influence of the speed of the compressor can be.

Moreover, the construction of the aforementioned coolant can sensors a sensor body with a coolant flow path cut, which is arranged within the pipeline, a Coolant chamber located on the inside of the sensor body Located above the coolant flow path section and in communication with the coolant flow path section manure, and a heat-sensitive component that is inside half of the coolant chamber to detect whether the coolant is in a liquid state within the chamber, pre see is include.

By using a coolant sensor with the above construction that has a heat sensitive component to capture sen whether a coolant that is in the coolant chamber above of the coolant flow path section occurs in a river condition, the coolant flow is not affected and a very precise detection of the coolant is possible.

The connection between the coolant chamber and the cooler Mittelflußwegabschnitt in the coolant sensor can over a constricting opening with a small diameter. With such a structure causes the narrowing opening with a small diameter a resistance to the river the coolant from the pipeline into the coolant chamber and from the coolant chamber into the pipeline. As a  The result can be instantaneous changes in the amount of coolant be suppressed in the coolant chamber such that the Detection accuracy can be further increased.

Fig. 1 is a schematic diagram according to a first game Ausführungsbei of the present invention showing the structure of a system.

Fig. 2 is a longitudinal sectional view showing the internal structure of a coolant sensor, which is used in the first embodiment.

FIG. 3 is a characteristic diagram showing the characteristic curves for determining a low-coolant condition in the first embodiment.

FIG. 4 is a flowchart illustrating a coolant shortage detection routine according to the first embodiment.

Fig. 5 is a longitudinal sectional view showing a coolant sensor according to a second Ausführungsbei game with the present invention.

Fig. 6 is a graph showing the characteristic curves of the coolant sensor sense current for each coolant level percentage.

There follows a description of the exemplary embodiments of the present invention with reference to FIGS. 1 to 5. First, a description is given of a first exemplary embodiment of the present invention with reference to FIGS. 1 to 4.

As shown in Fig. 1, a closed coolant circulation circuit 1 in an air conditioner for an automobile includes a pipe 2 which has a circulation circuit in which a coolant F, such as. B. ammonia or fleon gas, circulates with a compressor 3 , a condenser 4 and a United steamer 5 , which are successively provided along the pipe 2 in the direction of the circulation of the coolant F (indicated by arrow A in Fig. 1) , forms. The evaporator 5 is arranged so that its heat absorbing surface shows in the driving compartment of a vehicle (not shown in the figure), while the compressor 3 is connected to an engine 6 by a solenoid clutch 7 so that the rotation of the engine 6 is transmitted to the compressor 3 . The refrigerant F is compressed by the compressor 3 and is subjected to a subsequent phase change as it flows through the condenser 4 and the evaporator 5 , changing from a pressurized gas to a pressurized liquid and then to a gas done with low pressure. During the change from a liquid to a gas in the evaporator 5 , heat is absorbed from the travel compartment, thereby cooling the travel compartment internally.

In this case, the compressor 3 to the motor 6 by the solenoid clutch 7 is connected, the solenoid Kupp lung 7, the rotation of the motor 6 to the compressor 3 carries on. The solenoid clutch 7 takes z. B. by switching a switch for the air conditioning system (not shown in the figure) such that the rotation of the motor 6 is transmitted to the compressor 3 , whereby the compressor 3 is driven.

A collecting tank 8 is provided in the pipe 2 at a location between the condenser 4 and the evaporator 5 for temporarily holding the refrigerant F which has changed to a liquid state. The collecting tank 8 is provided with a viewing window 8 A, whereby the liquid state of the coolant F can be observed therein.

An expansion valve 9 is provided in the pipeline at a location between the header tank 8 and the evaporator 5 . The expansion valve 9 includes a pressure reducing valve which reduces the pressure of the liquid coolant F discharged from the sam meltank 8 to a predetermined pressure such that it flows in the direction indicated by an arrow A. The coolant F, the pressure of which has been reduced by the expansion valve 9 , evaporates as it flows through the evaporator 5 , passing into the gaseous state and subsequently being compressed again by the compressor 3 .

A coolant sensor 10 is seen in the pipeline at a location between the collecting tank 8 and the expansion valve 9 . The coolant sensor 10 , as shown in FIG. 2, basically includes a sensor body 11 , which will be described later, which forms the outer shape of the coolant sensor 10 , a coolant flow path portion 12 formed so as to pass through the sensor body 11 , A coolant chamber 17 , which is formed in the sensor body 11 at a location above the coolant flow path section 12 , and which communicates with the coolant flow path section 12 through a restriction opening 14 , and a thermistor 18 , which is provided within the coolant chamber 17 .

The sensor body 11 is formed at a right angle to form the parallel tubular shape of the exterior of the coolant sensor 10 and is substantially with the coolant flow portion 12 passing through the sensor body from the left side surface to the right side surface as shown in FIG. 2 . leads, and which is formed on the inner peripheral surfaces of its two ends with internal thread sections 12 A, 12 A ge, and with an opening 13 with a large diameter, which is formed at a location above the coolant flow path section 12 , which has an internal thread section 13 A has a portion on an inner peripheral surface of an entrance, and with the small diameter restriction opening 14 formed so that a connection exists between a bottom portion 13 B of the large diameter opening 13 and the aforementioned coolant flow path portion 12 .

The pipeline 2 is screwed into both internal thread sections 12 A in the coolant flow path 12 in such a way that the coolant F in the cooling circuit 1 communicates with the coolant flow path 12 .

Furthermore, a cover 15 is provided to cover the opening 13 with a large diameter. The cover 15 is cylindrically shaped and has an externally threaded portion 15 A, which is formed on an outer peripheral surface of the sen upper end, and it has an O-ring groove 15 B on the peripheral on an outer surface of the lower to the sen end is formed. A pair of openings 15 C, 15 C for electrical leads are drilled axially through the cover 15 in a diametrically spaced relationship. An O-ring 16 is in the O-ring groove 15 B fits, whereby the opening 13 is sealed with a large diameter.

The coolant chamber 17 is formed between the cover 15 and the bottom portion 13 B of the large diameter opening 13 . The coolant chamber 17 is formed by tapping the male threaded portion 15 A of the cover 15 into the male threaded portion 13 A of the large diameter opening 13 in the sensor body 11 , and communicates with the coolant flow path portion 12 through the above-mentioned restriction opening 14 .

A heat sensitive device in the form of the thermistor 18 is disposed within the coolant chamber 17 . The thermistor 18 is attached to the cover 15 by inserting connecting leads 18 A, 18 A into the respective respective openings for connecting leads 15 C in the cover 15 using stepped, cylindrically shaped sealing members 19 , 19 . The respective connection lines 18 A lead to the outside of the coolant sensor 10 and are connected to an externally attached warning device 33 , which will be described later. The thermistor 18 is positioned within the coolant chamber 17 so that it is spaced a predetermined height H above the bottom 13 B of the large diameter opening 13 .

In this embodiment, the thermistor 18 shows the like characteristics that when it is self-heated as a result of a current I flowing in the respective leads 18 A, the temperature rises and the resistance falls while with external cooling to it Time the temperature drops and the resistance value increases.

A speed detection sensor 31 is provided on the engine 6 . The speed detection sensor 31 detects the rotation speed of the engine 6 , whereby the rotation speed N of the compressor 3 connected to the engine 6 through the solenoid clutch 7 is detected.

A control unit 32 is provided for judging a shortage of the coolant filling the closed coolant circuit of the air conditioner based on signals from the various sensors. The input side of the control unit 32 is connected to the respective connecting lines 18 A of the coolant sensor 10 , which is provided in the pipeline 2 , and to the speed detection sensor 31 , while the output side to the warning device 33 , which is a device such. B. a lamp, is connected. Furthermore, a control program for assessing the coolant level, as shown in FIG. 4, is contained in the control unit 32 , and a characteristic table, such as. B. shown in Fig. 3, is together with In formations such. B. of reference speeds N0, which are calculated from the characteristic table, stored in a storage area 32 A of the control unit 32 .

The characteristic table shows the characteristic curves for each of the speeds N of the compressor 3 , the coolant level percentage B being shown on the horizontal axis and the output current I of the coolant sensor 10 being shown on the vertical axis.

Assuming that a coolant level percentage of 80% is a sufficient level percentage, then with reference to the previously mentioned illustration of FIG. 6, the area to the left of the characteristic curve B80 can be assumed to be an area with sufficient level and the area to the right as an area of the lack of coolant will. On the other hand, referring to the characteristic table of FIG. 3, it can be seen that on the line for the coolant level percentage of 80%, the sense current I is variable when the speed of the compressor 3 is less than 3000 rpm while the output current I for speeds above 3000 rpm remains constant.

In this case, after the line for the coolant level percentage B80 for judging whether the coolant level amount is sufficient or insufficient is set, with the present embodiment, when the speed N of the compressor 3 rises above 3000 rpm, one Judgment control using the detected current I cannot be carried out because judgment is only possible when the speed is less than 3000 rpm. Consequently, the speed of 3000 rpm is stored in the storage area 32 A as the reference speed N0.

With the characteristic table from FIG. 3, the assessment value with regard to the output current 1 through the line for the coolant fill level percentage B80 to the assessment values 1500, 11000, 12000, to 13000 for the respective speeds from N500, N1000, N2000, to N3000 of the compressor 3 set. Here, the respective evaluation values 1500 to 13000 are generally referred to as evaluation values IN.

The air conditioner according to the present invention is constructed in this way, while the judgment control corresponds to the coolant level according to the following description with reference to FIG. 4.

First, in step 1, the speed N, which corresponds to the speed of the engine 6 , of the compressor 3 is determined by reading the speed of the engine 6 by the speed detection sensor 31 . That is, the speed detection sensor 31 and the function of step 1 form the speed detection device.

In step 2, an assessment is made as to whether the Speed N is less than the reference speed N0 or not. If the judgment is "No" (N <N0), control returns go back to step 1 and execute the assessment control of the coolant level in step 3 and the subsequent steps are delayed.

On the other hand, if the judgment in step 2 is "Yes" (N <N0), the control goes to step 3 and the loading Judgment value IN corresponding to the speed N becomes from the Characteristics table read out. That is, the characters stik table and the function of step 2 include the Judgment value setting device.

In step 4, the current I of the thermistor 18 of the coolant sensor 10 is read, and in step 5, a judgment is made to determine whether the current I is larger than the judgment value IN or not.

If the judgment of step 5 is "Yes" (I <IN), then there is a shortage condition for the coolant F in the closed coolant circulation circuit 1, and as a result, the warning device 33 becomes effective to inform the operator of the coolant shortage to teach. Control then returns to step 7.

With the coolant level in such a condition, even if the pressure of the coolant F in the pipeline 2 fluctuates with the change in the speed N of the poet 3 , causing the liquid level of the coolant F inside the coolant chamber 17 upward and fluctuates downward, erroneous notification that a sufficient fill level exists despite the lack of the coolant can be reliably prevented after the judgment value IN is set to correspond to the rotational speed. Moreover, by delaying the evaluation control when the speed N is above the reference speed N0, a more accurate judgment of the level of the coolant F within the cooling circuit 1 is possible.

By setting the evaluation value IN, which corresponds to the speed N of the compressor 3 , from the characteristic table, an erroneous notification that an adequate state exists despite the insufficient fill level of the coolant F in the cooling circuit 1 can be reliably prevented in such a way that a faulty failure to warn the warning device 33 can be prevented. In addition, operation of the United poet 3 can be reliably prevented without load such that problems such. B. seizing or blocking of the compressor 3 can be prevented, and protection of the compressor 3 can be effectively controlled.

Furthermore, the thermistor 18 is arranged at a predetermined height H within the coolant chamber 17 . After the se height H in relation to the arrangement of the coolant sensor 10 and in relation to the arrangement of the pipe 2 of the closed coolant circulation circuit 1 can be set appropriately, a certain degree of freedom in setting the detection sensitivity for the coolant shortage is possible. After the detection characteristics of the coolant sensor 10 will be different, however, the characteristic table must be created again during the setting.

Next, with reference to FIG. 5, a description will be given of a second embodiment according to the present invention. A feature of this embodiment is the use of a sensor which is an improvement over a remaining amount of coolant sensor disclosed in Japanese Unexamined Patent Publication No. 3-199873. Components that have the same structure as those in the first embodiment are indicated by the same reference numerals and the description thereof is omitted.

The coolant sensor 41 essentially comprises a connection member 42 (described later) and a bolt-shaped sensor section 45 which is mounted on the connection member 42 .

The connection member 42 , which acts as a sensor body, which is provided in the pipe 2 , comprises a coolant flow path 43 , which leads through the connection member 42 from left to right, with threaded portions 43 A, 43 A, which in its left-hand and right-hand opening to Connection to the pipe 2 are formed, and a sensor mounting opening 44 which is bored axially from above so that it communicates with the coolant flow path 43 and which is formed with an internal thread on its inner peripheral surface.

The sensor section 45 has a metallic, tubular housing 46 with a closed bottom, which is formed with an external thread section 46 A on its outer peripheral front end for threaded engagement with the sensor mounting opening 44 , a bolt head section 46 B which is formed on the base end thereof, a cylin drically shaped sensor holder 47 , which is made of a resin material which is axially inserted into the housing 46 , a thermistor 48 , which serves as a heat-sensitive component, which is positioned at a front end of the housing 46 , with a Connection line 48 A, which is connected to a base plate 46 C of the housing 46 , and with a further connection line 48 B, which are connected to an output connecting rod 49 , a rubber sleeve 50 , which covers the open sides of the housing 46 together with the output connecting rod 49 , and a wire harness 51 for taking out a signal of thermistor 48 through sleeve 50 .

A restriction opening 52 with a small diameter is drilled in the base plate 46 C of the housing 46 , and a cooling medium chamber 53 is formed between the lower end of the sensor holder 47 and the base plate 46 C of the housing 46 . In the present embodiment, the thickness of the base plate 46 C of the housing 46 is 0.5 mm and the diameter of the restriction opening 52 is set within the range of 0.15-0.6 mm.

The coolant sensor 41 constructed in this way can operate in a manner similar to the previously described coolant sensor 10 . When the coolant F is sufficiently filled in the cooling circuit 1 , it flows into the coolant chamber 53 through the restriction opening 52 and covers and cools the thermistor 48 , thereby reducing the detection current of the sensor portion 45 . In the case of insufficient coolant, however, the rate of the cooling effect for the thermistor 48 is reduced and the detection current of the sensor portion 45 is increased after the coolant F does not enter the coolant chamber 53 .

Consequently, even with the coolant sensor 41 of this embodiment, a characteristic table that is approximately the same as the characteristic table shown in FIG. 3 can be obtained when it is used for judging control of the coolant shortage in the same manner as in the first Embodiment is used. Then, by performing the judgment control of the lack of the coolant as shown in FIG. 4, an operation effect that is the same as that of the first embodiment can be achieved based on this characteristic table.

Further, by the connection between the coolant flow path 43 and the coolant chamber 53 through the restric kung opening 52 with a small diameter of the movement of the coolant F between the coolant chamber 53 and the cooling mittelflußweg 43 is a flow resistance imposed. As a result, fluctuations in the liquid surface of the coolant F within the coolant chamber 53 due to the change in the rotational speed of the compressor 3 can be alleviated, and compared to the first embodiment, more accurate detection of the level of the coolant F is possible due to the additional functions of detecting a remaining amount of the coolant .

Moreover, with the present embodiment, the fitting and removal of the sensor section 45 can be easily performed after the coolant sensor 41 is assembled with the sensor section 45 which is threadedly attached to the connector 42 so that maintenance / replacement of the coolant sensor 41 is possible.

In the above embodiments, the detection of the rotation speed of the compressor 3 has been described, which is carried out by detection of the rotation speed of the engine 6 with the rotation speed detection sensor 31 . However, the present invention is not limited to the above method, and the rotational speed of the rotating shaft of the compressor 3 can be detected using a crank angle sensor or by providing a rotational speed detection sensor on the rotating shaft of the compressor 3 .

In addition, the warning device 33 in the above embodiments for warning of insufficient coolant level a lamp, and the warning is effective by turning on the lamp. However, the insufficient level warning can be performed using a program connected to a control unit and the like to stop the compressor 3 , and a buzzer or a synthetic voice can be input to the warning device 33 be installed in such a way that the warning is activated by the sound of the buzzer or a voice.

As described above, the liquid coolant suffers of the present invention in the pipeline fluctuations of pressure, as a result of the rotational speed of the Compressor, such that the liquid surface of the Coolant up and inside the coolant sensor sways down. The effect of this up / down Ten fluctuation can be adjusted by setting the judgment value so that it corresponds to the speed of the compressor, such alleviate that when the coolant level in a coolant deficiency condition, an incorrect assessment sharing of a sufficient condition reliably prevented can be. Accordingly, one can judge one sufficient or insufficient coolant level quantity regardless of the speed of the compressor be determined.

Furthermore, by using a coolant sensor, the coolant chamber that lies above the coolant flow path and communicates with it, and with a warmth sensitive component that is inside the coolant chamber is arranged, the coolant state with a high Ge accuracy can be detected without the coolant flow an influence is imposed.

Moreover, the connection between the coolant flow path and the coolant chamber through a restriction opening with small diameter the fluctuation of the cooling liquid surface up and down inside half of the coolant chamber due to the speed change of the  Compressor can be relieved so that the detection of the Coolant fill amount can be performed more accurately.

Claims (4)

1. A method for detecting a lack of coolant (F) in an air conditioning system, the air conditioning system comprising a coolant pipe ( 2 ), a compressor ( 3 ), a condenser ( 4 ) and an evaporator ( 5 ), which are successive along the coolant pipe ( 2 ) in a direction (A) of the circulation of the coolant by means of (F) in the coolant pipe ( 2 ), a coolant sensor ( 10 ; 41 ) in the coolant pipe ( 2 ) between the evaporator ( 5 ) and the condenser ( 4 ) is provided and produces an output value (I), the size of which depends on the amount of liquid coolant (F) which is in contact with the coolant sensor ( 10 ; 41 ), with the following method steps:

• - Detecting the speed (N) of the compressor ( 3 );
• - Determine whether the detected speed (N) is less than a reference speed (N0), which is predetermined by a coolant level, which is assumed to be a sufficient fill level, and above which an assessment of this coolant level (B) is not possible ;
• - Reading out an evaluation value (IN) from a table which contains the output values (I) of the coolant sensor ( 10 ; 41 ) depending on the coolant fill levels (B) for speeds (N) if the detected speed (N) is less than the reference speed ( N0), the assessment value (IN) being the initial value of the coolant sensor ( 10 ; 41 ) at a speed (N) and the predetermined coolant level (B);
• - Detecting the initial value (I) of the coolant sensor ( 10 ; 41 );
• - determining whether the detected initial value is greater than the assessment value (IN); and
• - Indication of a lack of coolant if the initial value (I) exceeds the assessment value (IN).

2. A device for detecting a lack of coolant (F) in an air conditioning system, wherein the air conditioning system comprises a coolant pipe ( 2 ), a compressor ( 3 ), a condenser ( 4 ) and an evaporator ( 5 ), which are successive along the coolant pipe ( 2 ) in a direction (A) of the circulation of the coolant by means of (F) in the coolant pipe ( 2 ), a coolant sensor ( 10 ; 41 ) in the coolant pipe ( 2 ) between the evaporator ( 5 ) and the condenser ( 4 ) is provided and produces an output value (I), the size of which depends on the amount of liquid coolant (F) which is in contact with the coolant sensor ( 10 ; 41 ), comprising:

• - A speed detection sensor ( 31 ) for the speed (N) of the compressor ( 3 );
• - A determination device ( 32 ), in which the signal of the speed detection sensor ( 31 ) can be entered, and in which it can be determined whether the detected speed (N) is less than a reference speed (N0), which is predetermined by a coolant level, which is sufficient fill level is assumed, and above which an assessment of this coolant fill level (B) is not possible;
• - A device ( 32 ), in which the signals of the speed detection sensor ( 31 ) and the coolant sensor ( 10 ; 41 ) can be entered, in which an evaluation value (IN) from a table that for speed (N) from output values (I) of the coolant sensor ( 10 ; 41 ) depending on predetermined coolant fill levels (B), can be read out if the determining device determines that the detected speed is less than the reference speed (N0), the evaluation value (IN) being an output value of the coolant sensor ( 10 ; 41 ) at a speed (N) and a predetermined coolant level (B), and in which the output value (I) of the coolant sensor ( 10 ; 41 ) is comparable to the evaluation value (IN); and
• - A device ( 33 ) for indicating a lack of coolant when the initial value (I) exceeds the assessment value (IN).

3. Apparatus according to claim 2, wherein the coolant sensor ( 10 ; 41 ) has the following features:
a sensor body ( 11 ; 42 ) with a coolant flow path ( 12 ; 43 ) which is in the coolant pipe ( 2 ) angeord net;
a coolant chamber (17; 53) in the sensor body (11; 42); (12; 43) is provided and in communication with the Kühlmit telflußweg is at a location above the Kühlmittelflußweges (43 12); and
a heat sensitive member ( 18 ; 48 ) provided within the coolant chamber ( 17 ; 53 ) for detecting whether the coolant in the coolant chamber ( 17 ; 53 ) is in a liquid state. 4. The device according to claim 3, wherein the connec tion between the coolant chamber ( 17 ; 53 ) of the coolant sensor ( 10 ; 41 ) and the coolant flow path ( 12 ; 43 ) through a restriction opening ( 14 ; 52 ) with one opposite the coolant chamber ( 17 ; 53 ) smaller diameter is made.